When data is communicated as radio frequency signals from a transmitting antenna to a receiving antenna, the radio frequency signal may travel directly to the receiving antenna. Alternatively however, a component of the radio frequency signal may be reflected from natural or man-made structures such as mountains, trees, or buildings. A first component of the radio frequency signal may be delayed from a second component of the radio frequency signal if the first component follows a different path to the receiving antenna from the path followed by the second component.
Because of an introduction of a phase shift to the first component of the radio frequency signal with respect to the second component, the first component may add or subtract from the second component. Such a subtraction can result in a reduction in signal level at the receiving antenna and is typically referred to as multi-path fading. Multi-path fading reduces the reliability of the data that is received at the receiving antenna after being transmitted from the transmitting antenna.
The problem of multi-path fading is especially acute when the receiving antenna is part of a moving vehicle. In that case, the occurrence of multi-path fading is dependent on the constantly varying position of the moving vehicle, and the level of multi-path fading is unpredictable.
Prior art communication systems deal with this problem of multi-path fading in moving vehicles. For example, U.S. Pat. No. 5,442,646 to Chadwick et al. shows a communication system that includes a comprehensive interleaving and encoding scheme to preserve the integrity of the data that is transmitted to a moving unit via the Subcarrier Traffic Information Channel (STIC). The scheme for a typical data structure used for transmitting data in a fading channel environment can be licensed from Mitre Tek, Inc., Reston, Va. Currently, Mitre Tek, Inc. is a private non-profit organization that is at least partially finded by the federal government and licenses the use of such a data structure for no charge. In addition, a description of this data structure is also available from Mitre Tek, Inc. as Memo No. D053-M-311 dated Sep. 26, 1994, and entitled "STIC Transmitter Software Description." In FIG. 1A, such a data structure 10 includes a superframe 12 that comprises a first data frame 14, a second data frame 16, a third data frame 18, a fourth data frame 20, and so on up to a two hundred and sixteenth data frame 22. Each data frame further includes a synchronization subframe 24, a first data subframe 26, a second data subframe 28, and so on up to a thirty sixth data subframe 30, and a low latency data subframe 32.
In order to assign a reliability factor to every data bit received, the communication system of Chadwick et al. includes at the transmitter end, an encoder, an interleaver, and a framing and synchronization circuit that inserts channel state information to every subframe within each data frame at the transmitter end. The receiving end then includes a channel state bit extractor, a correlation unit, a deinterleaver, and a decoder. The disclosure of U.S. Pat. No. 5,422,646 is incorporated herein by reference.
Although the communication system of U.S. Pat. No. 5,442,646 ensures high integrity in the data transmitted via a radio frequency channel in a fading channel environment, the complex interleaving, correlating, and encoding scheme of that system introduces a relatively high delay during the interleaving/deinterleaving, correlating, and encoding/decoding processes. However, time is of the essence for transmission of certain data, and the delay introduced by the complex interleaving, correlating, and encoding scheme is not acceptable.
For example, for in-vehicle traffic information systems, the Differential Global Positioning System (DGPS) data that is transmitted to a moving vehicle by a DGPS reference station is constantly being updated since the position of the moving vehicle is constantly changing. Because of this constant updating, the DGPS data must be transmitted to the moving vehicle with relatively low latency from the data being generated at the transmitter end to the data being available at the moving vehicle if the DGPS data is to accurately reflect the position of the moving vehicle.
The data structure of FIG. 1A available from Mitre Tek, Inc., Reston, Va. does include a low latency time frame 32 for carrying time-critical data. However, because the data within data structure 10 is used in a fading channel environment, the time-critical data can be corrupted to an unacceptable level if the time-critical data is simply inserted into the low latency data subframe 32 without some encoding/decoding scheme for maintaining acceptable data integrity.
Thus, a communication system that transmits time critical low latency data with relatively little delay yet with acceptable data integrity within a fading channel environment is desired.